Heat-treating iron steel body



Patented Dec. 13, 1949 FATENT OFFICE tWIsthniian Metals,

Inc., lloston; Mass, :ascorponationlof Massachusetts N0 Drawing. Originapplica't-ion August '16,

1944, Serial-No; 549,807; Divided and this applica'tion September 6Claims.

'21 This invention relates 1170 (the fabrication .of articles made .ofiron .or iron-alloyslbyl pressing rand. sintering startingtmaterial-which optionally has the final ,composition of the fabricated article.

.This-startingmaterial isu in loose powder .form.

An object of this-invention is to produce .a

smetal object of duplex structure such that one :componentlconsists :ofsubstantially carbon-free :iron and another component .consists ofsteel,

lone orl .bothtof these. components beingproduced thy. powder.metallurgy -methods. A further. object {is to heat-treat objects .ofsuch duplex structure so that the steel .component is hardened, whilesimultaneously the carbon! free iron component .is-softened orisunchanged in hardness.

-;-Another objectislto fabricate Lby, powder metal- .lurgyemethods asubstantially.carbonfree iron :rotatingbandtfor steels artilleryjshells, the rotating band being attached to the shell eitherduringfabrication of 'the band, or therea'fter, and heat- .treating thebandedv shell simultaneously to soften .the. iron'band, and leave the.shell ina .hardened condition. 7

Many usefulrmetal lartieles'may consist of'ia duplex iron-steelstructure. For .example,""artillery shells normally consist of ahardsteershelb ho'dy and a softrotating band which, uponenter- 'ing thegun barrel'becomes sufliciently deformed lto engage tightlythe"rifling"of"thebarrel. "The making of such rotating'orshellbandsfromiron powder is one "of the Limportantuses *for the method hereindescribed.

In'order tomake androrrsh'ell-band, the" start- "ing material maybea"loose"'and freelya-fiowi-ng iron powder.

'iThis charge'of'loose ironpowder subjected to cold-pressingata*relatively =light3-pressuremf -I5 :40"tons per"square inch.

This first "cold=pressed, body-isjsintered, -without using pressure"during "the F sintering, :at =l600" R-2000 F., during a"sintei'ingperiodof twenty minutesto' one*hour,-and= even up' tothreehours. As'atspecific examplethesmtering temperature may be 1660 R, and'thesintering period: may be "thirty minutes. another specific -example;:the 'sintering' temperature may be' 1 800" F. and the sinteringyperiodmaybe two 'hours.

This'sintering-is done in-an inert-*ornon-oxidiziingatmosphere.Forexample, "dry oxygen iree *hydrogen or dry" craeked -oxygen freeammonia gas provide a satisfactory. sintering atmosphere.for,sinte'ringan ironflcompact. "Whenrammonia .;gas is .crackd,nitrogen :aridjhydrogen aretproslduced. Dry and .oxygengfree nitrogenarid/or argon can also be used. ,Ifla.coldrpressedrsteel 28, 1946,Serial No. 700,166

briquette .prza mixtureof irons andoarbonis thusesintered;the.atmosphere maybea dry andoxy en :rfree unixturez of.1hydrogensandz methane .or Luther istabilizing igas, intermixed. in.suitable proportion ito prevent i.-.the .sintering atmosphere fromwearurizing .or .decarburizing thesbriguette vduring :ithefsintering.Thesinteredbodyzshould becooled in the sintering atmosphere, until the.sintered body willnotioxi'dize when-it 11810031286. in therair :or ';in;o.ther oxidizing *atmosphere. .Hence, the :sintered band is kept freefromoxidation.

.;After the first sintering the compact istagain cold .pressedat-,ahigher pressure to obtain the desired .density. .This ;higherpressure is prefer- ,l5 eablg atl-least-eapproximately .60 tons persquare inc Normally; a .second .sinteringloperation' is .use- 5315111afterqthersecond .coldqpress'ing ;in..order to sel-iminate or; .r,educethe workehardening which .20 -results -,from the second .cold pressing.and..,in

-..order to improve. the -;physical properties of the body..Thesecondsi'nter'ing isperformed under thersamegeneral. conditions astheifirst sintering.

The temperature. oflthe second 'sintering canbe i155 @1.5Q0.F.2000'i.F.,the sintering, period can'be -.twenty. minuteswto one hour ormore; and the sintered band is.. preferab1y allowed to cool slowly inthe sintering atmosphere to 400. F.-=500 'F.," as .previously stated.

The upperilin'iit of thesinteringtemperature nnay be. ashigh as 24'00F.; in sinteringeither iron or steel exceptthat' in sintering steel thesinterling JtemperatureJshould preferably "be kept at least I00 F. belowthe meltingpoint ofthestel, 3.5 which is being sintered. The meltingpoint of the ips'te'el depen'dsiupon its*carbon"-content.

Shell-iban'dsmadei rom said iron powder,-'ae- ,cjordingito themultiple-step --method previously fdescribedican be cold-'swaged intothe-groove'of .4011the shell or other iprojectile, using the same methodandapparatus which-"are'now used for col'd-swaging copper alloy'bandsintosaid groove. .Ihis co'ld-swaging causes -considerable work-"hardening-of' the iron band, and it maybe desir- 145 ,able" tosofteniii-"by suitable =heat=treatment For example prior to thecold-swagingthe iron -band may "have *a' hardness in e the -range Rockwell Y F $575, and after cold swaging a hardness 01 -Rockwell "F*82-96. Atypicalsteel-shell body has v.50 ,ahardnessof Rockwell' B 98-105.such-ashelltbody containing "a cold=swaged ironband ca-n' beiiheattreated by' heating it above the critical tem- .perature of thesteel .(abouti1400 F.) .which' is.alsdabove'theirecrystallizingtemperature or the cold-worked iron" band.Thisheating serves ,si

3 multaneously to austenitize the steel and allow recrystallization andgrain growth to take place in the iron. The banded shell may now becooled at a carefully selected rate as hereinafter disclosed which willrestore the steel to its original state of hardness, while the iron bandwill be found to have been softened to approximately it: pre-swaginghardness. Alternatively, the bandeL shell may be quenched in water,brine, oil, or any,

other suitable medium. Such cooling will produce in the steel a highhardness, e. g. Rockwell C 40-60, but the iron band will not be hardenedbut rather will be restored to its preswaging hardness. The hardness ofthe steel shell may then be lowered to its original value by tempering.

First alternative method As an alternative method of making a shellband,the band can be annealed after it has been cold-swaged into the grooveof the shell.

pressed in the mamier previously mentioned, at a pressure of -60 tonsper square inch to give bodies of substantially uniform densities offrom 5.5 to 7.3. After sintering at 1800 F. under the conditionspreviously stated, such bodies may have a hardness of from 10 to 80 onthe Rockwell is preferred. The steel shell-body and its iron shell-bandare then rapidly cooled from the annealing temperature, by a blast ofnon-oxidizing gas, to C.- C. This cooling period may be 5-20 minutes.This rapid cooling converts the steel material of the shell-body fromaustenite, largely to pearlite, without substantially affecting the pureiron or ferrite of the iron band. This treatment removes or reduces theworkhardening of the iron band which has resulted from the cold-swaging.1

Instead of using gas for said cooling, the

banded shell-body can be quenched from above its critical temperature,in water or oil which is ;at'a temperature of 20 C.-25 C. This criticaltemperature is about 1400" F., in an ordinary shell-body.

If the banded steel shell-body is thus quenched,

.the hardness of the banded steel shell-body is regulated by temperingor drawing in the well- ;known manner, by reheating the banded shellbodyto a temperature below its critical range, so that the strength andhardness of the steel shell-body are brought to the proper value. Suchtempering does not affect the hardness of the .pure iron shell-band. Inheat-treating the banded steel-body, the usual precautions arepreferably taken to prevent oxidation of the iron band.

-In this method, the shell-body and its attached band are simultaneouslyand equally heat-treat- .ed. The hardness of the final annealed band isabout -85 on the Rockwell F scale.

In a typical steel stock which is used for making a shell-body, thehardness of such steel .stock is between 9737-10437 on the Rockwell Bscale. The ultimate tensile strength of such steel stock is 99,700 lbs.per square to 128,800 lbs.

per square inch.

In this alternative method, the iron powder may be cold- In the finishedshell-body with its completed band, the Rockwell B hardness of the steelstock is 96-104, after final heat treatment in the case in which theband and shell-body are heat treated after the band. has been swaged onthe shellbody.

The composition of a typical steel shell-body may be 0.30%-0.40% carbon,1.15%-1.55% manganese, not more than 0.045% phosphorus, 0.075%-0.l50%sulphur, and 0.15%-0.30% silicon, the remainder being iron.

Second alternative method The loose iron powder is located in the grooveof the shell-body, and the first cold-pressing is performed on saidloose iron powder while it is so located, so that the first cold-pressedbody is formed in situ in the groove. This cold-pressing can be done byusing a die in which the shell-body is located. The pressure which isused in cold-pressing said loose and free-flowing iron powder in situ,may be tons per square inch. The banded steel shell-body is then heated,as for example at 1600 F.-1800 F. during a heating period of two hours.The higher temperature of 1800 F. is preferred. Since this heattreatmentsoftens thesteel body, its strength and hardness are adjusted byquenching and drawing, as in the first alternative method. A

single pressing and a single heat-treatment are Third alternatice methodIt may also be desirable to make a duplex article which consists partlyof relatively pure iron and partly of steel in which the steel componentis made by powder metallurgy methods. In this case the powder metallurgysteel component can be hardened by heat treatment while the ironcomponent is simultaneously softened or left unchanged in hardness. Sucha heat treatment may consist of a heating at a temperature which isusually within the range 1400" F.2000 F. followed by quenching in wateror oil and subsequent tempering, or the cooling may be more slowly at apredetermined rate suflicient to result in the desired hardness of thesteel component without subsequent tempering. During the heating it ispreferable to surround the duplex body by protective atmosphere in orderto prevent oxidation. If the heat treatment is relatively long (e. g. 15minutes or over), it is preferable to use an inert atmosphere such asdry nitrogen or one of the noble gases. The

-more usual protective atmosphere such as partially burned gases, orcracked ammonia. or hydrogen, or mixtures of hydrogen and a hydrocarbon,may be suitable when the heating period is relatively short. Howeverthese atmospheres react on iron or steel and during a prolonged heattreatment may either carburize the iron or.decarburize the steel or bothsince itis impossible for such atmosphere to be simultaneously inequilibrium with iron and steel.

In order to produce by powder metallurgy methods steel compacts suitablefor hardening by conventional heat treatments the following "proceduremaybe used.

amass According" to one m'ethod'which is within the scope ofmy-invention'j the iron powder is mixed with the proper proportion ofgraphite, a briquette is made of this mixture by cold-pressing at -40tons per square inch, and this briquette is then heated and thussintered in an atmosphere which prevents any injurious loss of thegraphite. during the sinteringoperation. Preferably this sinteringoperation .is carried out at a temperature of 200Q F. and for a periodof from one to three hours, in an atmosphere of methane and hydrogen.

The methane can be replaced by propane or other stabilizing gas.

It is desired to combine all the graphite with the iron powder. Hencetheproportion of graphite isthe desired final proportion of combined carbonin the final steelbody, The proportion of methane in the sinteringatmosphere depends upon the sintering temperature and the final proeportion of combined carbon which it is desired to have in the finalsteel product. The methane acts as a stabilizing gas, so that thegraphite combines with the iron, and not with the hydrogen.

The proportion of methane must not be excessive,

or else some of the methane will be decomposed, and the resultantliberated carbon will combine with the iron, to over-carburize the steelbeyond the desired limit. Hence the proportion of methane is selected sothat the briquette neither loses nor gains carbon during the sintering,and the proportion of combined carbon in the final steel material isdetermined by the proportion of graphite in the briquette.

As elsewhere explained herein, it is very important to provide asintering atmosphere during carburization, which is free from watervapor and oxygen. Hence the commercial hydrogen and stabilizing gasmixture is purified by removing the oxygen, and water vapor is alsoremoved, as further explained herein.

After the first sintering when the iron has combined with the graphitethe briquette is again cold pressed at a higher pressure to obtain thedesired density and then resintered. This higher pressure is preferablyat least approximately 60 tons per square inch.

Steel compacts made as described above may be combined with iron bodiesmade from wrought material or by powder metallurgy methods by any ofseveral methods for example the two may be joined by copper brazing,swaging, by riveting etc. Alternatively they may be joined during thepowder metallurgy operations of pressing and sintering.

This application is a division of copending application Ser. No.549.807, filed August 16, 1944, now Patent No. 2,411,073, for Makingproducts of iron or iron alloys.

I claim:

1. A method of making a duplex metal body of ferrous metal parts ofdifferent hardenability and wherein the separate or assembled parts intheir ultimate shape cannot be assembled or disassembled, comprising thesteps of preforming one part to its ultimate dimensions, preforming theother part to such dimensions as will permit its assembly with the onepart, assembling the parts, deforming the said other part into intimatemechanical engagement with said one part, said deformation of said otherpart being accompanied by severe internal work hardening, and thenwithdrawing the work hardening from said deformed other part by heattreating the duplex body at'atemperature which-is above the recrystallization temperature of the deformed part.

2. A method of making a duplex metal body of iron and steel partswherein the steel part is relatively harder than the iron-part andwherein the separate or assembled-parts in their ultimate shape cannotbe assembled or disassembled, comprising the steps ofpreforming thesteel part to its ultimate dimensions, preforming the soft iron part tosuch dimensions as will permit its assembly with the steel part,assembling the parts, deforming the iron part into intimate mechanicalengagement with the steelpart, said deformation of the iron part beingaccompanied by severe internal work hardening, and then withdrawing thework hardening from the deformed iron part by heat treating the duplexbody at a temperature which is above the recrystallization temperatureof the deformed iron part.-

3. The method of making a duplex metal body of interengageable preformediron and steel parts of different hardenability, the ultimate shape ofthe parts being such that prior to assembly at least one part must havedimensions different from its final dimensions to permit interengagementof said part with said other part, and that following assembly anddeformation to interengage the parts they cannot be disassembled withoutdestruction of one of the parts, comprising the steps of preforming thesteel part to its ultimate shape, preforming the iron part to suchdimensions as will permit assembling it with the steel part, assemblingthe parts, deforming the iron part into intimate mechanical engagementwith the steel part, said deformation of the iron part being accompaniedby severe internal work hardening, then Withdrawing the work hardeningfrom the iron part by heat treating the duplex body at a temperaturewhich is above the recrystallization temperature of the deformed.

iron part.

4. A method of making a duplex metal body of.

iron and steel parts wherein the steel part is: relatively harder thanthe iron part and wherein. the separate or assembled parts in theirultimate shape cannot be assembled or disassembled, comprising the stepsof preforming the steel part to its ultimate dimensions, preforming thesoft powder iron part to such dimensions as will permit its assemblywith the steel part, assembling the parts, deforming the soft iron partinto intimate mechanical engagement with the steel part, saiddeformation of the iron part being accompanied by severe internal workhardening, and then withdrawing the work hardening from the deformediron part by heat treating the duplex body at a temperature which isabove the recrystallization temperature of the deformed iron partwithout changing the hardness of the steel part.

5. A method of making a duplex metal body of iron and steel partswherein the steel part is relatively harder than the iron part andwherein the separate or assembled parts in their ultimate shape cannotbe assembled or disassembled, comprisin the steps of preforming thesteel part to its ultimate dimensions, preforming the soft powder ironpart to such dimensions as will permit its assembly with the steel part,assembling the parts, deforming the soft iron part into intimatemechanical engagement with the steel part, said deformation of the ironpart being accompanied by severe internal work hardening, and thenwithdrawing the work hardening from the deformed iron part by heattreating the du- 7 piex body at a temperature which is above therecrystallization temperature of the deformed iron part andconcomitantly increasing the hardness of the steel part.

6. A method of making a duplex metal body of iron and steel partswherein the steel part is relatively harder than the iron part andwherein the separate or assembled parts in their ultimate shape cannotbe assembled or disassembled, comprising the steps of preforming thesteel part to its ultimate dimensions, preforming the soft powder ironpart to such dimensions as will permit its assembly with the steel part,assembling the parts, deforming the soft iron part into intimatemechanical engagement with the steel part, said deformation of the ironpart being accompanied by severe internal work hardening, and thenwithdrawing the work hardening from the deformed iron part by heattreating the du- REFERENCES CITED The following references are of recordin the file of this patent: 1

UNITED STATES PATENTS Number Name Date 2,209,709 Weatherhead, Jr. July30, 1940 2,275,420 Clark et a1 Mar. 10, 1942' FOREIGN PATENTS NumberCountry Date 398,045 Great Britain Sept. 7, 1933

